In a typical cellular radio system, wireless terminals (also referred to as user equipment, unit nodes, UEs, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a radio base station (also referred to as a RAN node, a “NodeB”, and/or enhanced NodeB “eNodeB”). A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with UEs within range of the base stations.
Multi-antenna techniques can significantly increase capacity, data rates, and/or reliability of a wireless communication system as discussed, for example, by Telatar in “Capacity Of Multi-Antenna Gaussian Channels” (European Transactions On Telecommunications, Vol. 10, pp. 585-595, November 1999), the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein. Performance may be improved if both the transmitter and the receiver are equipped with multiple antennas to provide a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO. The LTE standard is currently evolving with enhanced MIMO support and MIMO antenna deployments. A spatial multiplexing mode is provided for relatively high data rates in more favorable channel conditions, and a transmit diversity mode is provided for relatively high reliability (at lower data rates) in less favorable channel conditions.
In a downlink from a base station transmitting from an antenna array over a MIMO channel to a wireless terminal, for example, spatial multiplexing (or SM) may allow the simultaneous transmission of multiple symbol streams over the same frequency from different antennas of the base station antenna array. Stated in other words, multiple symbol streams may be transmitted from different antennas of the base station antenna array to the wireless terminal over the same downlink time/frequency resource element (TFRE) to provide an increased data rate. In a downlink from the same base station transmitting from the same antenna array to the same wireless terminal, transmit diversity (e.g., using space-time codes) may allow the simultaneous transmission of the same symbol stream over the same frequency from different antennas of the base station antenna array. Stated in other words, the same symbol stream may be transmitted from different antennas of the base station antenna array to the wireless terminal over the same time/frequency resource element (TFRE) to provide increased reliability of reception at the wireless terminal due to transmit diversity gain.
Due to its potential to substantially improve the spectral efficiency of a wireless communication system, very-large-scale MIMO (VL-MIMO) systems, devices and methods with at least eight and, in some embodiments, tens or hundreds of antennas per cell site have recently received much attention in both academia and industry. See, for example, Marzetta, “Noncooperative Cellular Wireless With Unlimited Numbers of Base Station Antennas”, IEEE Trans. on Wireless Communications, Vol. 9, No. 11, pp. 3590-3600, November 2010, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein. A base station or access node equipped with a large number of antennas can transmit multiple independent data streams to one or many geographically separated UEs simultaneously over the same frequency band by exploiting the different spatial signature unique to each UE's channel response. This has the potential of substantially improving the downlink system capacity of a cellular network. Moreover, such a base station or access node can also utilize the large number of antennas at the receiving end to average out the noise and to cancel many interferers, which can lead to substantial increase in uplink capacity.
The potential dramatic increase in the capacity of a MIMO system through the use of increasing number of antennas has long been promised by the random matrix theory. For example, for a nR by nT MIMO channel H with IID entries, the Marcenko-Pastur law implies that the (single-user) open-loop capacity normalized by the number of receive antenna, as the number of transmit antennas nT tends to infinity and the ratio of transmit and receive nT/nR→β, converges to a constant as given by
                    1                  n          R                    ⁢      C        =                            1                      n            R                          ⁢        log        ⁢                                  ⁢                  det          ⁡                      (                          I              +                              SNR                ⁢                                                                  ⁢                                  HH                  H                                                      )                              ->                        ∫          0          ∞                ⁢                              log            ⁡                          (                              1                +                                  SNR                  ⁢                                                                          ⁢                  λ                                            )                                ⁢                                          ⁢                                    p              β                        ⁡                          (              λ              )                                ⁢                      ⅆ            λ                                where                    p        β            ⁡              (        x        )              ≡                                        (                          1              -              β                        )                    +                ⁢                  δ          ⁡                      (            x            )                              +                                                                                    (                                      x                    -                                                                  (                                                                              β                                                    -                          1                                                )                                            2                                                        )                                +                            ⁢                                                (                                                                                    (                                                                              β                                                    +                          1                                                )                                            2                                        -                    x                                    )                                +                                                          2            ⁢            π            ⁢                                                  ⁢            x                          .            Hence, the open-loop capacity grows linearly with the number receive (or transmit) antennas in this case.
VL-MIMO may be of particular interest at high frequency bands (e.g. 60 GHz band) where many antenna elements can be packed within a small amount of space due to the small radio wavelength at the high frequency band. In this case, beams with high directivity can be formed with a relatively small aperture size. This can provide improved spatial resolution in resolving different scatterers surrounding the UEs.
In order to attain the potential gain achievable by VL-MIMO, knowledge of the MIMO channel state information generally is desirable. MIMO channel state information may be used by the receiver to perform the demodulation of transmitted data symbols. MIMO channel state information may also be used at the transmitter to properly shape the transmit signal to improve Signal to Interference and Noise Ratio (SINR) at the receiver.
In many existing wireless cellular communication systems, pilot symbols are transmitted for each antenna or antenna port over radio resource elements that are non-overlapping in time and in frequency with (in other words, orthogonal to) those pilot symbols transmitted for other antennas or antenna ports. For example, in the current release (Rel. 10) of LTE, known Reference Signals (RSs), or pilot symbols, are transmitted at various time instants and frequencies for different antenna ports, as shown in FIG. 1. Specifically, FIG. 1 illustrates cell-specific downlink reference signals that may be used in LTE systems for one, two and four antenna ports, as described, for example, in the textbook by Dahlman et al. entitled “3G Evolution: HSPA and LTE for Mobile Broadband, Second Edition”, 2008, pp. 325-328, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein. Using these known RSs, the receiver can then estimate the channel response from each transmit antenna to each receive antenna across all times and frequencies. In this pilot design methodology, the respective RSs for different antennas may need to be non-overlapping with each other to reduce or prevent the estimated channel response for each antenna element from being distorted by the channel response of other antenna elements. This undesired phenomenon is sometimes referred to as pilot contamination.
Unfortunately, as the number of antennas grows large, for example eight or more antennas, the same pilot transmission methodology used in LTE with orthogonal pilot patterns for different antenna elements may use up much or even all of the radio resource elements for pilot transmission, leaving little or no radio resources for data transmission.